The time loops are dependent on proximity to Sisi and time of death.
- In the first movie, the time loop latched onto Tree because of all the people in relatively close proximity to Sisi, she had the least amount of time left to live. Once she prevented her death, the loop transferred to Ryan since it was his turn to die. But when Tree was knocked into the alternate timeline, the loop returned to her. If true, this makes for some serious Fridge Horror since it means that DARPA (and possibly even Tree herself) are repeatedly killing Danielle in the name of a science experiment.
after Tree goes back to her own universe, Alt!Carter starts dating his universe's TreeBecause without losing her mother, Tree didn't become bitter or bitchy. While that wasn't enough to keep Carter from dating Danielle (although we never find out how long; it could be a very new relationship), he's just spent a day with Tree being her best, most awesome self, and he seemed very impressed, even before she more or less told him that 'her' Carter was the reason she was going back, and that rather epic kiss. After that, at the very least he'd want to get to know his universe's version of Tree. Alt!Carter hasn't changed from the person that Tree fell for originally, and as long as Alt!Tree's close to the person that Tree became after her Character Development (and why wouldn't she be?) it seems like it would be a sure thing that they'd fall for each other all over again.
as the first movie plot-referenced Groundhog Day and the second movie plot-referenced Back to the Future Part II, the third will plot-reference The Terminator.
- The first movie introduced the time loop from Groundhog Day, the second introduced parallel universes from Back to the future II. What other movie franchise is famed for time shenanigans? It would also follow the natural path of Tree's emotional progression: the first movie had her coming to terms with the present (she realized she wasn't a good person, and resolved to change for the better), the second movie was about her reconciling with her past (her mother's death). What else is there to do but safeguard her future?
- It could even be resolved by Tree having to go back to September 17, in order to interfere with Sisi and start the original time loop in the first place.
in number 3, Carter is the one in the time loop, becoming a true Deuteragonist
- Ryan showed at the start of 2U that Tree wasn't the only person who could time-loop. While the Stinger seemed to set up Danielle as the looper for number 3 during DARPA's experiments... um, who the hell would knowingly pick a spoiled, bitchy, Brainless Beauty sorority chick as the centerpiece for a highly confidential, very strange experiment that could have permanent effects on the body and mind? Actual, controlled experiments have properly prepared and screened volunteers for that sort of thing, even under-the-table Government experiments. note Given Tree's expression as she tells the DARPA agent she knows the perfect person for their experiment, Danielle sitting up in bed and screaming could just be Tree's Imagine Spot.
- this could even lead to the choice for the Babyface Killer: an Alt!Carter. 2U had a murderous Alt!Ryan trying to kill the original Ryan while he was time-looping, who disappeared (presumably sent back to his own universe once Sisi was fixed). If Sisi malfunctions and sends Carter through a time-loop, what about an Alt!Carter doing the same thing for a different reason: Alt!Carter lost Tree (possibly to another killer version of Lori), and now he's in a universe where she's alive, he'll do whatever he has to in order to stay with her, which means getting rid of Original!Carter. This would even tie into Tree's dilemma from 2U.)
in #3, the Babyface Killer will be Stephanie Butler
- In #1 Lori killed Tree over her affair with Gregory Butler. In 2U, Gregory Butler was trying to kill Lori to cover up their affair, and Stephanie was helping him in order to avoid scandal. In the alternate ending for #1, after she fixes the time loop, Tree was murdered by Stephanie as revenge for sleeping with her husband. Following this progression, #3 might have Stephanie finding out about Tree's (now-ended) affair with Gregory because of the scandal of Lori's attempted murder and subsequent death at the end of #1, and deciding to kill Tree in revenge.